Methods and systems for monitoring and controlling electroerosion

ABSTRACT

A method for monitoring machining in an electroerosion assembly having a power supply and an electrode arranged across a gap from a workpiece, includes measuring a voltage at a point in a voltage waveform after a time delay t d  of one half of a pulse width of the voltage waveform. The measurements are repeated for multiple pulses of the voltage waveform to obtain multiple voltages, each of the voltages corresponding to a point in respective pulses. The voltages are averaged to obtain an average voltage, which is compared with at least one threshold voltage, to determine whether the machining is in control. A control signal is generated if the comparison indicates that the process is not in control, the control signal being configured to regulate an operating parameter of the power supply, and the control signal is supplied to the power supply, if generated.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.10/996,218, Yuanfeng Luo et al., entitled “Methods and systems formonitoring and controlling electroerosion,” which patent application isincorporated by reference herein in its entirety.

BACKGROUND

The present invention relates in general to electroerosion, and morespecifically to systems and methods for monitoring and controlling anelectroerosion process.

Electrochemical machining (ECM) and electrical discharge machining (EDM)are conventional processes for machining material in objects such as gasturbine components. ECM processes typically pass an electrical currentin the gap between an electrode(s) and a workpiece for precision removalof amounts of material on the workpiece to achieve a desired finalconfiguration thereof with substantially smooth surfaces. EDM processescirculate a dielectric liquid between an electrode(s) and a workpiece,and electrical discharges are generated in the gap between the electrodeand the workpiece. EDM is used, for example, to drill small film coolingholes through the surfaces of turbine rotor blades and nozzle vanes.

Both ECM and EDM processes use electrical current under direct-current(DC) voltage to electrically power removal of the material from theworkpiece. However, in ECM an electrolyte (an electrically conductiveliquid) is circulated between the electrode(s) and the workpiece forpermitting electrochemical dissolution of the workpiece material, aswell as cooling and flushing the gap region therebetween. In contrast,EDM processes circulate a nonconductive (dielectric) liquid in the gapto permit electrical discharges in the gap for removing the workpiecematerial. As used herein, the term “electroerosion” should be understoodto apply to those electromachining processes that circulate anelectrolyte (electrically conductive liquid) in the gap between theelectrode(s) and the workpiece, these processes enabling a high rate ofmaterial removal and reducing thermal damages to the workpiece.

Beneficially, electroerosion processes provide for quicker machining andhave higher efficiencies as compared to other electromachining methodsin various applications, such as, blisk roughing and machining, forexample. Typically, in processes utilizing an electroerosion assembly, avoltage potential is generated across a gap between an electrode and aworkpiece to be machined, resulting in an electrical discharge in thegap. According to physics of the electroerosion process, when themachining electrode (cathode) approaches the workpiece (anode) surfaceseparated by the gap, an electrical discharge occurs through the gap dueto the voltage across the electrode and the anode workpiece. The gap,which constitutes a machining zone, is filled with a liquid electrolytemedium with moderate to low electrical conductivity, and the gap allowsfor the flow of electrolyte, which removes eroded particles from the gapin addition to providing a suitable medium for electrical discharge orsparking for electroerosion. A “normal” electrical discharge across thegap results in desirable machining of the workpiece. An “abnormal”discharge on the other hand results in undesirable errors in machiningwhich may have a direct repercussion on the surface finish of themachined workpiece. In some cases, the workpiece or the electroerosionassembly may be damaged due to short-circuiting because of a lack of aneffective control.

Such errors can be avoided by timely and accurately monitoring ofdischarge patterns, detecting abnormal discharges and accordingly takingcorrective measures. However, the systems currently employed in EDM formonitoring discharge patterns are generally insufficient and/orunsuitable for monitoring electroerosion processes and typicallygenerate errors in detecting a discharge type. The errors may amount toincorrectly classifying normal discharges as abnormal and vice versa,which makes the electroerosion process susceptible to the risksmentioned above. For example, many conventional EDM assemblies employ anignition delay detection method to determine whether a discharge isnormal or abnormal. In the ignition delay detection method, dischargeswith an ignition delay are considered normal, whereas those without anignition delay are considered abnormal. However, as noted above,electroerosion processes use electrolytes instead of the dielectricliquids used for typical EDM processes. Accordingly, for electroerosionprocesses, a number of the pulses without ignition delay are normaldischarges, and the conventional ignition delay detection methodimproperly classifies many normal discharges as abnormal, when used tomonitor electroerosion processes.

Accordingly, there exists a need for accurate detection andclassification of voltage discharge in electroerosion processes.Consequently, electroerosion processes and systems with accuratemonitoring and control are also desired.

BRIEF DESCRIPTION

An aspect of the present invention resides in a method for monitoringmachining in an electroerosion assembly having a power supply and anelectrode arranged across a gap from a workpiece. The power supplyenergizes the electrode by applying a potential difference between theelectrode and the workpiece during multiple pulse-on periods. The methodincludes measuring a voltage at a point in a voltage waveform after atime delay t_(d) of one half of a pulse width Δτ of the voltagewaveform. The measurements are repeated for multiple pulses of thevoltage waveform to obtain multiple voltages, each of the voltagescorresponding to a point in respective pulses. The voltages are thenaveraged to obtain an average voltage, and the average voltage iscompared with at least one threshold voltage V_(th), to determinewhether the machining is in control. A control signal is generated ifthe comparison indicates that the process is not in control, the controlsignal being configured to regulate an operating parameter of the powersupply, and the control signal is supplied to the power supply, ifgenerated.

Another aspect of the invention resides in another method for monitoringmachining in an electroerosion assembly having a power supply and anelectrode arranged across a gap from a workpiece. A voltage is measuredat a point in a voltage waveform after a time delay t_(d) of one half ofa pulse width Δτ of the voltage waveform. The measurement is repeatedfor multiple pulses of the voltage waveform to obtain multiple voltages,each of the voltages corresponding to a point in respective pulses, thepulses corresponding to a window. The voltages are compared with athreshold voltage V_(th) to classify the pulses as a normal or anabnormal discharge pulse. A control signal is generated if thecomparison indicates that a predetermined number of abnormal dischargepulses are present in the window. The control signal is configured toregulate an operating parameter of the power supply, and is supplied tothe power supply, if generated.

Another aspect of the inventions resides in an electroerosion assemblyincluding an electrode configured to machine a workpiece, located acrossa gap from the electrode. The machining is achieved upon application ofa potential difference across the electrode and the workpiece. A powersupply is configured to energize the electrode is also included in theelectroerosion assembly that further includes a controller configured tomeasure a voltage at a point in a voltage waveform after a specifiedtime delay t_(d) of one half of a pulse width Δτ of the voltagewaveform. The controller is also configured to repeat the measurementfor multiple pulses to obtain multiple voltages, each of the voltagescorresponding to a point in respective pulses. The controller thencompares the voltages with a threshold voltage V_(th), to determinewhether a machining process is in control. The controller generates acontrol signal if the comparison indicates that the machining process isnot in control, the control signal being configured to regulate anoperating parameter the power supply, and supplies the control signal tothe power supply, if generated.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a schematic illustration of an electroerosion assembly,according to an embodiment of the invention;

FIG. 2 illustrates a flow diagram of a method for monitoring and controlmachining using the electroerosion assembly;

FIG. 3 illustrates a flow diagram of another method for monitoring andcontrol machining using the electroerosion assembly;

FIG. 4 is a plot illustration characterizing various types of dischargesin machining using an electroerosion assembly;

FIG. 5 is a waveform illustrating an example of an applied controlsignal;

FIG. 6 is a waveform illustrating another example of an applied controlsignal; and

FIG. 7 is a waveform illustrating yet another example of an appliedcontrol signal.

DETAILED DESCRIPTION

FIG. 1 illustrates an electroerosion assembly 100 according to anembodiment of the invention. The electroerosion assembly 100 includes anelectrode 10 configured for machining, arranged across a gap 12 from aworkpiece 14 to be machined. The gap 12 and at least a portion of theelectrode and the workpiece are submerged in an electrolyte medium (notshown in the figures). A power supply 20 is configured for generatingelectric discharges that machine the workpiece. Typically, thedischarges cause particles at a machining site of the workpiece toseparate from the workpiece, thereby machining the workpiece 14. Thepower supply 20 energizes the electrode 10 by applying pulses of apotential difference ΔV to generate such discharges, in the gap 12across the electrode 10 and the workpiece 14. The multiple appliedpulses having a pulse width Δτ result in an applied voltage waveformhaving multiple pulse-on and pulse-off states. In response to theapplied voltage waveform, the discharges in the gap 12 generate avoltage waveform that represents the voltage in the gap 12, and thevoltage waveform comprises multiple discharge pulses. It is appreciatedthat the applied waveform (having applied pulses) and the voltagewaveform (having discharge pulses, or “pulses”) are distinct. A servocontroller 30 is provided for controlling mechanical movements in theassembly 100, including controlling the gap 12, the relative movement ofthe electrode 10 and the workpiece 14 for alignment. A controller 40 isprovided for monitoring and controlling electroerosion processes ormachining using the electroerosion assembly 100. The controller 40 iscoupled to the power supply 20, the servo controller 30, the electrode12 and the workpiece 14.

The controller 40 is generally configured to, among other functions,direct the power supply 20 to apply pulses of potential difference ΔVbetween the electrode 10 and the workpiece 14, measure a voltage acrossthe gap 12, generate selectable time intervals, initiate the measurementafter the passage of an interval of time (or a time delay), averagevoltage values, compare voltage values, and generate control signals toregulate the power supply 20.

The controller 40 may include, without limitation, a microprocessor oranother computational device, a voltage measurement device, a timingdevice, a pulse generation device, a voltage comparison device, a datastorage device, among others. All such devices are well known in the artand any such suitable device may be used without deviating from thescope of the invention.

FIG. 2 illustrates a method for monitoring machining in theelectroerosion assembly 100, according to an embodiment of theinvention. According to the method S100, after starting (S110) themachining, a voltage is measured (S120) at a point after a specifiedtime delay t_(d), of at least half the applied pulse width Δτ from apulse-on state of an applied pulse of the applied voltage waveform. Thetime delay t_(d) may be varied, for example, to two-thirds of theapplied pulse width Δτ, and such variations are included within thescope of the present invention. The measurement of the voltage as aboveis repeated (S130) for multiple pulses of the voltage waveform, eachmeasurement being completed after a specified time delay from a pulse-onstate of applied pulses of the applied waveform. The number of multiplevoltage measurements may be set to a predetermined number, such as four(4) voltage measurements, for example. The multiple voltages, someasured, are averaged (S140) to obtain an average voltage, V_(avg). Theaverage voltage V_(avg) is then compared (S150) to a threshold voltageV_(th) to determine whether the machining is in control. In anembodiment, the determination includes indicating the machining is “notin control” if V_(avg) is less than V_(th). If the machining isindicated as being “not in control,” a control signal is generated(S160) to regulate at least one operating parameter of the power supply20. Operating parameters of the power supply include durations ofpulse-on or pulse-off states of the applied pulses, voltage of theapplied pulses, current supplied in the applied pulses, among others.The control signal, if generated, is supplied (S170) to the powersupply.

It is appreciated that the averaging may be done using any of thevarious averaging techniques including, but not limited to, simpleaveraging, point-by-point averaging among others. For example, apredetermined number of voltage measurements as above may be repeated,and these measurements may be averaged. A predetermined interval, whichinterval may include a number of pulses is referred to as a window (alsoindicated by numeral 302 in FIGS. 5-7). In one example, the windowcorresponds to six (6) pulses and thus to an interval equal to six (6)applied pulse widths (6*Δτ). This example is illustrative, and theinvention is not limited to a window corresponding to any specificnumber of pulses. In another embodiment, the window is a flying window,which is a variable interval configured to include a variable number ofpulses. The flying window advantageously allows for a flexiblemonitoring of the process, by increasing or decreasing the number ofpulses measured.

The control signal may be configured to regulate the power supply 20 ina number of ways. According to one embodiment as illustrated by FIG. 5,the control signal instructs the power supply 20 to increase a durationof a pulse-off period 300 between applied pulses to a longer duration ofpulse-off period 310. An instance 304 denotes a time at which thecontrol signal is supplied, and similar instances are also indicated forthe FIGS. 6 and 7 respectively. In the embodiment of FIG. 5, the controlsignal supplied at the instance 304, and the control signal isillustrated as causing the pulse-off period duration 300 between twosimultaneous pulses, before the control signal was supplied, to beincreased to a longer pulse-off period duration 310, after the controlsignal was supplied. According to another embodiment as illustrated byFIG. 6, the control signal instructs the power supply 20 to decrease theduration of a pulse-on period 320 of the applied pulses to a decreasedduration of pulse-on period 330. According to yet another embodimentillustrated by FIG. 7, the control signal instructs the power supply 20to modify peak current density supplied to the electroerosion assembly100. The modification includes decreasing the peak current densitysupplied to the machining site when the peak current reaches a peakcurrent threshold limit indicated by the numeral 340. The values ofvarious pulse-on or pulse-off durations, threshold limits, as discussedmay be modified as per specific machining conditions and desiredmachining quality, among other parameters. Various such values willreadily occur to those skilled in the art and are included within thescope and spirit of the invention.

Advantageously, these techniques assist in restoring the machiningprocess to a normal condition, and also decrease the energy supply tothe machining site thereby minimizing the damage to the electrode andworkpiece. According to a yet another embodiment, the control signalinstructs the power supply 20 to turn off when the machining process isout of control. It is appreciated here that any one of the strategies asabove, alternate strategies, or various combinations of the abovestrategies may be employed, by configuring the control signalappropriately.

At certain instances, the machining process may go “out of control,”which may mean a continued occurrence of “not in control” dischargesthat do not resume to a normal condition even after applying a control.Such “out of control” instances may occur for example, on electrodedistortion, cyclic arcing, bad electrolyte flushing conditions,incorrigible arcing due to electrolyte shortage, among others. Suchinstances may also be controlled by the strategies as discussed above,for example, by turning off the power supply. The machining may then beresumed after an appropriate interval of time, or after another suitableremedial action has been taken.

It is further noted that the discharge pulses may be classified asnormal or abnormal to indicate the normalcy of the discharge. The twocategories “normal” and “abnormal” correspond to desirable andundesirable machining performance, respectively. In certain instances,multiple closely spaced abnormal discharge pulses may correspond to a“not in control” process. In certain other cases, multiple continuousabnormal discharge pulses may correspond to an “out of control” process.Under normal machining conditions, the number of normal pulses issubstantially greater than the number of abnormal pulses. In a normaldischarge, the energy released by the discharge is used as explosiveenergy to remove workpiece material as particles. Examples of normaldischarges include, but are not limited to, normal discharge with anignition delay 210 and normal discharge without an ignition delay 220,examples of which are shown in FIG. 4. In an abnormal discharge, whichincludes an “arc” state or a “short-circuit” state of the discharge, ahigh amount of current may be generated, generating a high amount ofenergy. Most of the energy released in an abnormal discharge, however,is converted into excess heat energy that may damage the electrode 10 orthe workpiece 14, and in some cases cause an undesirable deformation ofthe workpiece surface. Examples of abnormal discharges are shown, forexample in FIG. 4, that include an arc with an ignition delay 230, anarc without an ignition delay 240, an arc and discharge combined 250 (ashort circuit and a deformed wave pattern) among others. A thresholdvoltage V_(th) is marked on FIG. 4, and a discharge voltage valuegreater than V_(th) at the point of measurement (after a time delayt_(d)) indicates a normal discharge, whereas a discharge voltage valueless than V_(th) indicates an abnormal discharge. The threshold voltagevalue V_(th) is chosen for differentiating between a normal and anabnormal discharge and may be determined experimentally for a particularelectroerosion assembly setup. The threshold voltage value V_(th) variesbased on the machining conditions, for example the material beingmachined and the electrolyte employed. In one embodiment, the thresholdvoltage value V_(th) is about 14.3 Volts. It is appreciated here that inFIG. 4, voltage level indicated by V_(th) is indicative and not meant tobe to scale.

A method S200 embodiment for monitoring machining in an electroerosionassembly is illustrated in FIG. 3. Once the machining is started (S210),a voltage is measured (S220) at a point after a specified time delayt_(d), of at least half the applied pulse width Δτ from a pulse-on stateof an applied pulse of the applied voltage waveform. The time delayt_(d) may be varied, for example, to two-thirds of the applied pulsewidth Δτ, and such variations are not limiting to the present invention.The measurement of the voltage as above is repeated (S230) for multiplepulses of the voltage waveform, each measurement being done after aspecified time delay from a pulse-on state of applied pulses of theapplied waveform. The multiple pulses being measured in S230 correspondto a window, which is a predetermined interval. In one embodiment, thewindow is a flying window. The multiple measured voltages correspondingto multiple pulses are then compared (S240) to at least one thresholdvoltage V_(th) to classify each of the discharge pulses in the window asnormal or abnormal. As discussed, the determination includes indicatingthe pulse as abnormal if the measured voltage for that pulse is lessthan V_(th). If the comparison indicates a pre-determined number ofpulses in the window as abnormal, a control signal is generated (S250)to regulate at least one operating parameter of the power supply 20. Inan example, the predetermined number of abnormal pulses required togenerate a control signal is set to six (6). However, this number isonly an example, and the window is not restricted to any specific numberof pulses. The control signal, if generated, is supplied (S260) to thepower supply.

The control signal is configured to employ any one or a combination ofcontrol strategies by regulating an operating parameter of the powersupply 20, as also discussed earlier. It is noted here that a singlepulse voltage measurement may be measured for generating a controlsignal. It is further noted that there may be multiple threshold voltagevalues for comparison, similar to V_(th). For example, a secondthreshold value V_(th2) (not illustrated in the figures) may beincluded, and a pulse is classified as abnormal if the measured voltageis greater than V_(th2), and various such suitable voltage thresholdvalues will occur to those skilled in the art, and do not limit theinvention.

Use of the techniques and systems as above advantageously allows for awell-monitored and controlled machining process. Among other benefits,accidental damages to the workpiece, which at times may be irreversibleor untreatable, are minimized, thereby reducing costs and in certaincases cycle times of manufacture of various components. The methods andsystems as disclosed also provide an improved, online monitoring systemfor electroerosion processes.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

1. A method for monitoring machining in an electroerosion assemblyhaving a power supply and at least one electrode arranged across a gapfrom a workpiece, the electrode being energized by the power supply thatapplies a potential difference ΔV between the electrode and theworkpiece during a plurality of pulse-on periods, said methodcomprising: measuring a voltage at a point in a voltage waveform for theelectroerosion assembly after a specified time delay t_(d) of at leastabout one half of a pulse width Δτ of the voltage waveform; repeatingthe measurement for a plurality of pulses of the voltage waveform toobtain a plurality of voltages, each of the voltages corresponding to apoint in respective ones of the pulses, wherein the pulses correspond toa window; comparing each of the voltages with at least one thresholdvoltage V_(th) to classify each of the pulses as a normal or an abnormaldischarge pulse; generating at least one control signal if thecomparison indicates that at least a predetermined number of abnormaldischarge pulses are present in the window, the at least one controlsignal being configured to regulate at least one operating parameter ofthe power supply; and supplying the at least one control signal to thepower supply, if the control signal has been generated.
 2. The method ofclaim 1, wherein the window is a flying window, said method furthercomprising selecting a window size.
 3. The method of claim 2, whereinthe electroerosion assembly is a pulsed electroerosion assembly, whereinsaid measuring begins at the time delay interval t_(d) after a pulse-onstate and ends at a pulse-off state, and wherein the at least onecontrol signal is an instruction to the power supply selected from thegroup consisting of an instruction to increase a duration of at leastone of a plurality of pulse-off periods, an instruction to decrease aduration of at least one of the pulse-on periods, an instruction tomodify a peak current density provided to the electroerosion assembly,an instruction to turn off the power supply and combinations thereof. 4.An electroerosion assembly comprising: at least one electrode configuredto machine a workpiece across a gap upon application of a potentialdifference ΔV across said electrode and the workpiece; a power supplyconfigured to energize said electrode; and a controller configured to:measure a voltage at a point in a voltage waveform for theelectroerosion assembly after a specified time delay t_(d) of at leastabout one half of a pulse width Δτ of the voltage waveform, repeat themeasurement for a plurality of pulses of the voltage waveform to obtaina plurality of voltages, each of the voltages corresponding to a pointin respective ones of the pulses, compare the voltages with at least onethreshold voltage V_(th), to determine whether a machining process is incontrol, generate at least one control signal if the comparisonindicates that the machining process is not in control, the at least onecontrol signal being configured to regulate at least one operatingparameter of said power supply, and supply the at least one controlsignal to said power supply, if the control signal has been generated.5. The electroerosion assembly of claim 4, wherein the plurality ofpulses corresponds to a flying window, and wherein the averagingcomprises a point-by-point averaging.
 6. The electroerosion assembly ofclaim 4, wherein said controller is further configured to determine thatthe machining process is not in control if the average voltage is lessthan the at least one threshold voltage V_(th).
 7. The electroerosionassembly of claim 4, wherein said controller is configured to comparethe voltages by: averaging the voltages to obtain an average voltage;and comparing the average voltage with at least one threshold voltageV_(threshold), to determine whether the machining process is in control.8. The electroerosion assembly of claim 15, wherein said controller isconfigured to compare each of the voltages with at least one thresholdvoltage V_(th) to classify each of the pulses as a normal or an abnormaldischarge pulse, and wherein said controller is configured to generateat least one control signal if the comparison indicates that apredetermined number of abnormal pulses are present in the window.
 9. Amethod for monitoring a machining process in an electroerosion assemblyhaving a power supply and at least one electrode arranged across a gapfrom a workpiece, the electrode being energized by the power supply thatapplies a potential difference ΔV between the electrode and theworkpiece during a plurality of pulse-on periods, said methodcomprising: measuring a voltage at a point in a pulse of a voltagewaveform for the electroerosion assembly after a specified time delayt_(d) from a pulse-on state of the pulse, the time delay t_(d) being atleast about one half of a pulse width Δτ of the voltage waveform;comparing the measured voltage with at least one threshold voltageV_(threshold), to determine whether the machining process is in control;generating at least one control signal if the comparison indicates thatthe process is not in control, the at least one control signal beingconfigured to regulate at least one operating parameter of the powersupply; and supplying the at least one control signal to the powersupply, if the control signal has been generated.